The present invention relates to a safety switching apparatus for safe disconnection of an electrical load, in particular a load used in an automated installation. More particularly, the invention relates to safety switching apparatuses for safely shutting down a machine or installation in response to a safety requirement signaled by an emergency-off buttons, a light curtain, a guard door or other safety-related signaling devices.
DE 100 11 211 A1 discloses a prior art safety switching device in the form of a compact device unit which is generally intended for installation in a switchgear cabinet of an automated installation. Such a safety switching device typically has an (at least largely) predefined and fixed functional scope. Such safety switching devices exist in particular for evaluation of emergency-off buttons, guard doors, switching mats, two-hand switches, limit position and other position switches and other safety-related signaling devices. They typically include an evaluation and control unit designed for evaluation and/or monitoring of one or more signaling devices of a specific type. Depending on the signaling device, the evaluation and control unit produces a control signal, by means of which an electrical power supply path to the load can be interrupted in a fail-safe manner when required.
In addition, so-called safety controllers exist, whose functional scope is freely programmable in wide ranges, such as safety controllers, which are marketed by the present assignee under the brand name PSS®. The present invention relates in particular to compact safety switching devices which are relatively simple and low cost in comparison to such programmable controllers. However, the invention can also be used for safety controllers, for failsafe remote I/O units, or for other types of safety switching apparatuses.
In conventional safety switching devices, the at least one switching element typically is a positively guided relay, that is to say an electromechanical switching element having a plurality of make contacts and at least one break contact. The break contact and the make contacts are coupled to one another via a mechanical positive drive such that the break contact and the make contacts cannot be closed at the same time. The make contacts are normally closed by the evaluation and control unit during operation of the safety switching device and they are used to interrupt the electrical power supply path to the electrical load when required due to a safety function. A signal is typically fed back via the break contact to the evaluation and control unit, so that the latter can monitor the switching position of the make contacts on the basis of the positive drive. This makes it possible for the evaluation and control unit to detect when a make contact has become welded, and is stuck in its closed (or open) switch position. Because of this characteristic, positively driven relays have been proven to be very highly suitable in the field of safety switching devices and have been widely used for many years. However, positively driven relays are quite expensive and quite bulky.
DE 100 11 211 A1 proposes a safety switching device in which at least two electronic switching elements are used in order to interrupt the electrical power supply path to the load. In particular, transistors are proposed as switching elements. This allows a safety switching device to be produced in a smaller form, and at a lower cost. However, the safety switching device known from DE 100 11 211 A1 differs from most conventional safety switching devices not just in the use of transistors instead of positively guided relays. A further difference is that the transistors in the safety switching device each produce a potential-related output signal while, in contrast, safety switching devices having positively guided relays typically provide floating outputs. The latter means that the safety switching device does not per se produce an output signal, but only either passes on or does not pass on a potential that is connected from the outside. In contrast, the safety switching device known from DE 100 11 211 A1 produces “its own” output potential which is related to the ground for the safety switching device.
Safety switching devices with floating outputs (that is to say with positively guided relays as switching elements) are widely used in practice because this technique has been in use for many years. For spare-parts compatibility reasons, it is desirable to continue to have safety switching devices with floating outputs. Furthermore, floating outputs have the advantage that they can switch currents, voltages and frequencies in the load circuit over a very wide variation range. In contrast, the switching capacity of the safety switching device DE 100 11 211 A1 is restricted by the characteristics of the transistors used. Consequently, there is still a need for safety switching devices with floating outputs.